Pad printing is an indirect printing technology of hollow rubber head and has become a leading printing method printing and decoration of surface of various materials now.
1、Principle: the technical process is simple—firstly, the designed patterns are etched on the printing slab and ink over the etched plate; secondly, most of ink is printed on the printed materials by means of transferal through silica gel head. 2、 Method of inking over the etched plate: there are many methods of inking over the etched plate. Firstly, spray the ink over the etched plate and then scrape off the excessive ink by a retractable scraper. In such case, the solvent from the ink remained in the etched area volatizes and shapes a colloid surface and then the silica gel head lowers onto the etched plate and sucks up the ink. 3、 Sucking of ink and printing of products: after sucking up most of ink, the silica gel head moves up and then the ink of this layer will volatize another part and the surface of the remained wet ink is conducive to the close binding of the printed material and silica gel head. The forming of the silica gel head can produce a rolling action so as to discharge the excessive air on the surface of etched plate and ink as possible. 4、
Cooperation between ink and silica gel head during production:
the most ideal case is that all ink on the ET is transferred to the printed material and the printing of etched plate is easily affected by air, temperature and static electricity during the production (approximately 10 micrometer- or 0.01mm-thick ink adhesion was transfer printed onto the ground substance). If the volatizing speed and solution rate is just controlled under the equilibrium condition during the entire process from etched plate to transfer-printing silica gel head and to ground substance, then the printing is successful. If the ink volatizes too quickly and the ink will be dried up before sucking; if the ink volatizes too slowly, then the colloid state cannot be formed on the ink surface and the silica gel head is not easily adhere to the ground substance. 5、
Etched plate:
the leading etched base plates include as below: 1. heavy steel plate; 2. thin steel plate; 3. plastic light-polymerized plate (light sensitive). As to any image, they are formed by means of photographic process. Firstly, the positive film is produced out of the photographed object by means of emulsion and then the positive film is put onto the plate which has been applied with the photosensitive coating. The image-free areas will harden when such plate is exposed to ultraviolet ray; secondly, the photoactive substance in the image areas will be washed off (by means of the mixed water and alcohol if the plastic light-polymerized plate is applied) and then such plate will be aired. The hi-quality silica gel head production device is cheap and it will cost about 3,000 yuan as required by customers. As to metal plate, after the photoactive substance in the image areas is washed off, the plate shall be put into the ferric chlorate, muriatic acid or other echants for etching, while such operation shall be handled carefully. It depends on the typographer as to whether the heavy or thin steel plate or plastic light-polymerized plate will be adopted—heavy steel plate is fit for the printing the most, but the printing quality is poorer; unlike the heavy steel plate, thin steel plate will be usually precoated with a layer of emulsion and such practice is very practical. However, the plastic light-polymerized plate is the best one among the abovementioned three in terms of the printing quality, but it is the worst one in terms of the production. The depth and quality of etching is very critical—if the depth is improper, then the image effect is poor. If any defective poor steel plate is applied, then the printed output will demonstrate the defect accordingly. 6、 Silica gel transferal head: The transferal head is made from the flexible silica gel and can be formed in various shaped, which is used to produce patterns on etched plate and transfer and print such patterns onto the ground substance. The ink-sucking capability of transferal head and its printing quality is subject to the effect of the following factors:
1.
Shape of transferal head;
2.
Hardness of transferal head;
3.
Surface condition of transferal head;
；4.
Manufacturing material for transferal head.
The essential shape of transferal head tends to be round, oblong and square and these shapes are suitable to the rolling of patterns when the surface of transferal head contacts with the silica gel head first and then with the printed material. The size and shape of patterns and the degree of rolling decides what transferal head is selected. The principle for such selection is that the harder the transferal head is, the better the transferal effect of ink is, whereas such hardness depends on the amount of silicon oil added into the rubber—little silicon oil shall be added into the ink with solvent. The desired surface of transferal head shall be very smooth and dim and a very good effect can be obtained when the transferal head is cleaned with the sheer fabric soaked with solvent. If any sign of wearing appears on the transferal head after 50,000 printings, the transferal head shall be replaced. Such replacement shall be handled carefully, so that the surface of transferal head will not be damaged. 7、
Ink： The ink with high pigment content, quick setting resin and quick vaporizing solvent is necessary for silica gel transferal head to print on slices. Since there are many materials that can be printed with this technology, there are various inks for option. The ink for printing by silica gel transferal head is expensive due to its less consumption, whereas the ink for silk screen printing is generally unsuitable to the transferal head printing due to its slow setting and 5% pigment content.
8、
Sealing Oil Cuplike Pad Printing Machine： The model of sealing-oil ink duct means an alternative option in relation to the open ink duct. For such model, the ink is contained in an up-side-down mounted container and contacts with the etched plate by means of pressure. The scraper for such container clears away the ink from the etched plate. Being sealed, such container cannot volatize the solvent, while the condition of ink is maintained stably so as to extend the cycle of such printing machine. Although such model seems having solved the problem of ink stability, it is still a very important issue as to how to blend the ink and many factors will also affect the volatilization of solvent. The printer shall be operated quietly carefully and prudently, or the etched plate will be worn and the container will spray the ink onto the areas beyond the etched plate. It is very costly to replace with a new ink duct. Such system is more and more applied to the automatic production lines. 9、
Key points of printing by silica gel head： From negative plate to galvanic etched plate: the positive-image negative plate to be duplicated is exposed before the blank plate with light-polymerized galvanized layer and then you can obtain the galvanic etched plate. From rubber head to galvanic etched plate: here the silica gel head transferal head is pressed onto the galvanic etched plate and the ink shapes the pattern to be printed and the transferal head is raised.
From silica gel head to the finished product: then the silica gel head is pressed onto the fixed ground substance and the ink is discharged at that time.
Taking the finished product from the silica gel head: the silica gel head is uplifted from the printed material, the scraper is used to scrape the ink on the galvanic etched plate and then the silica gel head can suck up the ink only from the part to be printed next time.
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